1. Field of the Invention
This invention relates to a Stirling cooler comprising a compressor and a displacer apparatus, and more particularly a centering device for centering a displacer of the displacer apparatus, which reciprocates in response to changes in pressure of working fluid depending upon operation of the compressor.
2. Description of the Prior Art
Generally, as shown in FIG. 1 of the accompanying drawings, a Stirling cooler comprises a compressor 10 including a cylinder 12 and a piston 14 reciprocating within the cylinder 12 to effect compressing operation, and a displacer apparatus 20 including a cylinder 21 and a displacer 23 slidably mounted within the cylinder 21 to reciprocate in response to changes in pressure of working fluid depending upon operation of the compressor 10.
The cylinder 12 of the compressor 10 has cooling fins 11 formed on its outer periphery and outlet 19 formed centrally in its top for passage of the working fluid. A crankcase 13 is connected to a lower end of the cylinder 12, and a crankshaft 15 coupled to a drive motor 16 is disposed within the crankcase 13 and connected to the piston 14 via a connecting rod 17. The piston 14 slidably mounted in the cylinder 12 has a piston ring 18 fitted on an outer periphery thereof to be brought into air-tight engagement with the inner periphery of the cylinder 12.
The displacer apparatus 20 further comprises a freezer 22 disposed on the top of the cylinder 21. The cylinder 21 has a step potion 21a provided internally of the middle portion thereof and defining a central bore of a reduced diameter, and a hole 21b formed in side wall thereof to be connected to the outlet 19 of the compressor 10.
The cylinder 21 of the displacer apparatus 20 comprises an expansion space 24 defined above the displacer 23 reciprocating in the cylinder and a work space 25 defined between the step portion 21a and the displacer 23. Thus, the volumes of the expansion and work spaces 24, 25 are variable with the reciprocating movement of the displacer 23. In addition, a buffer spacer 26 effecting a damping function is provided below the step portion 21a to be in communication with the work space through the bore of the step portion.
Further, a displacer rod 27 is connected to the lower surface of the displacer 23 and extends downwardly through the central bore provided by the step portion 21a of the cylinder 21. A seal ring 28 is fitted on the outer periphery of the middle portion of the displacer rod 27 to function as a seal between the rod and the step portion 21a, thereby preventing leakage of the working fluid from the work space 25 into the buffer space 26. In addition, a regenerator 29 effecting a heat exchange function is disposed in the inner hollow space of the displacer 23.
The hole 21b formed in the side wall of the cylinder 21 of the displacer apparatus 20 is connected to the outlet 19 of the cylinder 12 of the compressor 10 through a tube 30, which serves as a passage for flow of the working fluid between them.
Operation of the Stirling cooler thus constructed will now be described below.
First, the working fluid is introduced into the respective spaces of the Stirling cooler at the same pressure so that the pressure of the fluid is constant throughout the interior of the cooler. Thus, the fluid in a space provided in the cylinder 12 of the compressor 10 by the piston 14 disposed therein has the same pressure as those in the expansion and work spaces 24, 25 formed above and below the displacer 23 of the displacer apparatus 20.
The buffer space 26 of the displacer apparatus 20, into which one end of the displacer rod 27 is extended, serves as a gas spring when the displacer 23 reciprocates in response to variations in pressure of the working fluid.
Therefore, when the fluid pressure in the compressor 10 varies with the reciprocation of the piston 14 in the cylinder 12, which is effected through the crankshaft 15 and the connecting rod 17 by the operation of the drive motor 16, there correspondingly occur changes in the pressure of the fluid in the expansion and work spaces 24, 25 of the displacer apparatus 20. As a result, the difference between the pressures applied to the upper and lower surfaces of the displacer 23 causes repeated reciprocation of the displacer in the cylinder 21. At this time, a damping action is provided by the pressure of the fluid in the buffer space 26 applied to the lower end of the displacer rod 27.
The reciprocation of the displacer rod 27 leads to variations in pressure of the fluid in the work and buffer spaces 25, 26. Curves of the pressure variations are shown in FIG. 2. As can be seen in the graph, since the degrees of the compression and expansion of the working fluid are increased according to the repeated reciprocation of the working fluid are increased according to the repeated reciprocation of the piston 14 of the compressor 10, the pressure of the working fluid in the work space 25 of the displacer apparatus fluctuates greatly from the initial introduction pressure of the fluid. To the contrary, since the upward and downward movements of the displacer rod 27 cause a small change in the volume of the buffer space 26, the pressure of the fluid in the buffer space 26 fluctuates slightly as compared with the pressure in the work space 25.
Further, during the operation, a curve of the variations in actual pressure of the fluid in the work space 25 in the compression step has a distinctly different configuration from that in the expansion step. As shown in the graph of FIG. 3, the curve in the compression step has a peaked configuration.
In the Stirling cooler having the characteristics as set forth above, as shown in FIG. 1, to prevent leakage of the working fluid from the work space 25 into the buffer space 26, the seal ring 28 is disposed on the outer periphery of the displacer rod 27. However, a sealing arrangement comprising only this simple seal ring 28 fails to perfectly prevent the leakage, but provides slight leakage. Particularly, since the leakage of the working fluid is proportional to a square of the pressure of the fluid, the higher the pressure, the greater the amount of the leaked fluid. Therefore, since variations of the pressure in the buffer space are small, so that the pressure may be considered to be substantially constant, or substantially the same as the initial introduction pressure, the leakage of the fluid from the work space into the buffer space becomes greater than that from the buffer space into the work space. According to repetition of such a phenomenon, the fluid is continually introduced from the work space into the buffer space, thereby increasing the pressure in the buffer space. The increase of the pressure in the buffer space causes a phenomenon in which the displacer 23 effects reciprocation while being continually displaced upward. As a result, a reciprocation position of the displacer 23 is deviated from the initial position, so that operational performance of the apparatus is lowered.